This research is aimed at the development of an all-hydro-carbon spin-label for elucidating intermolecular interactions in the organized environments of lipid bilayers and biological membranes. The reporter group will be a trimethylenemethane biradical, whose rotational motions in such media can be expected to be highly anisotropic and can be followed by electron spin resonance spectroscopy of the triplet ground state. The advantages of such a system include minimal perturbation of the lipophilic aggregate by polar moieties and increased sensitivity as compared to other systems now in use.